1. Field of the Invention
This invention concerns method of marking a pipeline location, in general. More specifically, it relates to an improved method of making a positive identification of a location marker which is applied to a pipeline, and through which pipeline a pig travels with instruments carried thereon for making measurements in the pipeline.
2. Description of the Prior Art
Because of the long distances involved in pipelines and the need to make surveys of conditions therein which must be done by sending a pipeline pig through the line, it is important to be able to accurately determine locations of such pig during its travel through the line. The problem has been recognized by a U.S. Pat. No. 3,409,897 to Bosselaar, et al, which issued Nov. 5, 1968. That patent dealt with the transmission of a pipeline pig through the line for detecting leaks in the line. However, the markers employed (to determine specific locations along the pipeline) made use of an ultrasonic marker that transmitted a particular ultrasonic frequency signal into the pipeline. The signal would be picked up and recorded by the pig as it travelled past. But the leak signals would be in the same ultrasonic frequency range, and it was necessary to rely on the regularity or at least on the expected intervals from the known locations in order to distinguish the marker signals.
Consequently, it is an object of this invention to provide a method of marking and detecting a particular pipeline location, which method is able to positively identify such location whether or not there is noise or leak signals existing in or around the pipeline location.